Here is another AdSense test I spotted at DigitalPoint Forums. I believe these will be called horizontal images. Google seems to be placing an image in horizontal leader-board ads. Here is a screen shot cropped, to fit better on this site, pulled from the forum.
Notice the image on the left within the AdSense unit. This does not appear to be a manipulation of the ad, especially since the one who posted the image said it came from his site and he did not put the image there.
Jennifer Slegg reported back in June 2006 that Google was testing this images in vertical banners. Jen said:
AdSense has launched a new beta test called Vertical Images, where an image would take the place of an ad within an AdSense ad unit. These images - which are generic, and not company-specific - act similar to an ad link unit, linking to a page related to the ads and image that appeared in the ad unit.
Publishers are allowed to click the images themselves (just be careful not to accidentily click the ad below it!) but you cannot click on any of the ads that appear on the results page, just as how publishers can click on ad links keywords but not on any of the results.
I have not seen this tested since.
It is funny that they are testing this since Google Officially Disallows Images Near AdSense Ads in December 2006.
Forum discussion at DigitalPoint Forums.